7.2.2 Gene Pools & Allele Frequency

Key Definitions

• Gene Pool: The complete set of all alleles for all genes in the individuals of a population.
• Allele Frequency: The proportion of a specific allele within the gene pool of a population.

Understanding Allele Frequency

• Allele frequency is expressed as a decimal or percentage.
• It represents how common an allele is compared to other alleles of the same gene in the population.
• Example: If the A allele frequency is 0.6, then 60% of the alleles for that gene in the population are A alleles.

The Hardy-Weinberg Principle

The Hardy-Weinberg principle is used to predict the frequency of alleles, genotypes, and phenotypes within a population when it is not evolving.

For this to apply, certain conditions must be met:

1. Large population size to minimise genetic drift.
2. No mutations that change the allele frequencies.
3. Random mating between individuals.
4. No migration (gene flow) into or out of the population.
5. No selection for or against specific alleles. Hardy-Weinberg Equation:

$$p^2 + 2pq + q^2 = 1$$

Where:

• p = frequency of the dominant allele.
• q = frequency of the recessive allele.
• p² = proportion of homozygous dominant individuals.
• 2pq = proportion of heterozygous individuals.
• q² = proportion of homozygous recessive individuals.

Example Calculation

If 9% of a population has a homozygous recessive phenotype:

6. q² = 0.09, so q = 0.3.
7. p + q = 1, so p = 0.7.
8. You can calculate genotype frequencies using the equation:

• Homozygous dominant: $p^2 = 0.7^2 = :highlight[0.49]$ or 49%
• Heterozygous: $2pq = 2(0.7)(0.3) = :highlight[0.42]$ or 42%.
• Homozygous recessive: $q^2 = :highlight[0.09]$ or 9%